The use of the Ethernet® in the industrial field is currently growing. At manufacturing sites, factory-automation (FA) networks have been constructed, which are communication networks that connect FA devices, such as a programmable logic controller (PLC), an inverter, and a sensor. In an FA network, in order to execute high-speed and high-accuracy motion control, a relay device is required to transfer a frame, which is sensitive to delay fluctuations, with a fixed delay time. Examples of a fixed-delay frame required to be transferred with a fixed delay include a frame to be used for time synchronization between FA devices.
In a general relay device, a store-and-forward transfer control method is used, in which the entire frame is accumulated and then output. According to the store-and-forward transfer method, “Priority Queuing” described in Non Patent Literature 1 is used, in which transfer frames are categorized into classes and the priority is defined for each of the classes. By thus preferentially transferring a high-priority frame, it is possible to reduce a transfer delay time. However, even by using the “Priority Queuing” described above, the store-and-forward method still cannot eliminate delay jitter. This is because, in the store-and-forward method, when a high-priority frame is input during the transmission of a low-priority frame, the high-priority frame needs to wait until transmission of the low-priority frame is completed, thereby causing fluctuations in the transfer delay time for the high-priority frame.
As long as a transfer conflict occurs between a high-priority frame and a low-priority frame at an output port, there is a possibility for delay jitter, determined on the basis of a maximum frame length and a transfer speed of the low-priority frame, to be added to a transfer delay time for the high-priority frame. Therefore, in order to eliminate delay jitter of a high-priority frame, it is required to avoid a transfer conflict between a high-priority frame and a low-priority frame at the output port. In a demultiplexing device including a plurality of input ports, a transfer conflict between high-priority frames may occur. Therefore, the demultiplexing device is also required to avoid a transfer conflict between high-priority frames.
The aforementioned technique to reduce delay jitter of a high-priority frame when a transfer conflict occurs between a low-priority frame and a high-priority frame is disclosed in, for example, Patent Literature 1. In this technique, when a high-priority frame is input to a relay device during transmission of a low-priority frame, the relay device interrupts transmitting the high-priority frame by inserting the low-priority frame thereinto, so as to reduce delay jitter of the high-priority frame. The relay device adds a control code indicating an insertion position at the beginning and the end of the interrupting high-priority frame to be transmitted, and then it transmits this frame.
Further, Patent Literature 2 discloses a technique of a communication device, in which when a transfer conflict between a low-priority frame and a high-priority frame occurs, the communication device stops to transmit the low-priority frame and preferentially transmits the high-priority frame. The communication device stores the low-priority frame in a retransmission buffer concurrently with transmitting the high-priority frame. After transmitting the high-priority frame, the communication device retransmits the low-priority frame for which transmission has been stopped due to the transfer conflict.
Furthermore, Patent Literature 3 discloses a technique of transferring a high-priority frame more preferentially than a low-priority frame by operation of a time-slot control. A network device transfers a high-priority frame with a low delay repeatedly at a predetermined constant cycle. Therefore, a transfer interval to transfer a high-priority frame at a timing in synchronization with a transmission cycle is provided in a certain cycle, which makes high-priority frames transfer with a low delay and makes low-priority frames transfer in the remaining available time other than the transfer interval.